Hubbell Tips & News - April 2020

reducing the hydrophobic properties significantly or completely. Temporary loss in surface tension can leave some polymer materials vulnerable to tracking before hydrophobic properties are regained. The ideal polymer however would need to have a permanently hydrophobic surface or have a high level of resistance to leakage currents. ESP™ displays characteristics of hydrophobicity, but the material does not rely on this characteristic alone to ensure long term reliability. The original design goal for ESP™ was to meet or exceed the capability of existing porcelain designs in terms of resistance to surface leakage current and/or dry band arcing activity. The materials that have been developed by Hubbell have succeeded in achieving this goal. Without good performance in these key areas, the chances of field failures increase significantly. COMPONENTS OF A RELIABLE COMPOUND Compounds suitable for electrical insulation can consist of 10 or more ingredients which can be broken down to three major categories, shown in Figure 1. These include the base polymer, fillers which can make up nearly 50% of the total compound, and active additives. Compounding of an elastomer with fillers and additives to achieve the desired results for a given application is critical. The components are carefully selected to enhance field performance and ease of manufacture.

reinforcing and extending. Reinforcing fillers can improve tensile strength, modulus, tear strength and abrasion resistance of a compound. An extending filler is a loading or non-reinforcing material used to enhance desirable properties such as electrical characteristics and flammability resistance. Alumina Trihydrate (ATH, or Hydrated Alumina) acts as an extending filler in ESP™ which further improves the compound’s already good electrical characteristics. As an additional benefit when the ATH is overheated it goes through an endothermic reaction in which it rehydrates to release water. This leaves behind the inorganic component but, in the process, cools the surface making it more resistant to damage due to dry band arcing on its surface. Additives Additives support the manufacturing processes by protecting the chemical bond between fillers and elastomer during vulcanization. These materials make up a small percentage of the overall compound but help manufacturers achieve consistency in the final product. The combination of these three components results in a well-rounded polymer that can stand up to environmental factors which might otherwise deteriorate most polymers. TESTING FOR LONG TERM RELIABILITY The next step after defining the characteristics required of an ideal polymer housing material was to develop an appropriate test protocol. Good polymer compounds used for high voltage insulation should be tested for the ability to resist tracking, erosion, corona, and ultra-violet (UV) radiation exposure to ensure long term reliability. The section below provides a high-level overview of the key test procedures defined to achieve the previously mentioned characteristics. Detailed information is found in Polymer Compounds used in High Voltage Insulators [1]. The testing regime, outlined in Table 1, allows various materials to be evaluated and led to the optimum material selection for electrical insulation applications.

Figure 1: Typical Composition of a Polymer Compound

Base Polymer The base polymer that makes up a compound is called an elastomer. The two referenced in this paper and widely recognized are EPDM and Silicone. Selection of the base compound is only the first step in designing a reliable polymer compound for electrical applications. ESP™ is an EPDM based compound which was formulated over many years of research starting before the transition from porcelain to polymer. Fillers Fillers are used to support the base elastomer of a compound and typically make up 40-60% of a compound. There are two types of fillers which are classified as

Table 1: Lab Performance Requirements of Hubbell Polymer Compounds

Test

Minimum Passing Criteria

Tracking and Erosion

15,000 cycles

Ultraviolet (QUV)

8,000 hours

Corona Cutting

1,000 hours

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